Universal Serial Bus (USB) ports are commonly found on a variety of portable devices such as laptop computers, tablets, mobile phones, MP3 players, etc., and are also provided on desktop computers, automobile dashboard consoles, etc., to provide interconnection for serial communications between devices. In addition, the USB standards provide for charging capability by which mobile phones or other portable devices can be operated using power provided through a USB cable to the device. This power feature of the USB system also advantageously allows battery-powered devices to be charged using power provided from a connected USB-compatible device. Dedicated charging equipment is available, for example, having multiple USB ports for charging various portable devices, even where no serial communication is needed between the charging equipment and the device. The original USB implementations provided for charging at 5V with a charging current limited to 1 A, and subsequent revisions to the standards (e.g., USB 3.0, 3.1, etc.) provide for fast charging at higher levels such as using charger voltages of 5V, 12V, 20V and charging current levels of 1 A, 3 A and 5 A, thus accommodating up to 100 W charging. However, mismatching between USB charging sources and charged devices can lead to product damage and/or the inability to minimize charging time. Accordingly, there is a need for improved USB charger apparatus and electronic devices by which the charging power level can be maximized without damage to the charger or charged electronic device.